Many prior art patents are directed to hydraulically actuated disc brakes which include mechanical means to actuate the brake as a parking brake. For example, the U.S. Pat. No. 3,827,534 to Brooks discloses a brake actuator assembly which incorporates a spring-actuated sliding bolt which mechanically applies the parking brake. Other prior art patents teach brake actuator constructions which utilize spring biased pistons for providing standby fluid pressure to apply the brake force in response to a loss of fluid pressure in the brake system. However, none of the aforementioned patents teach the concept of using a separate fluid circuit to actuate a fluid-actuated service brake as a parking brake. Additionally, there are prior art patents which disclose a bellows-type actuator replacing the larger, heavier and more costly piston-type actuator.